Stabilized semiconductor nanocrystals

ABSTRACT

A semiconductor nanocrystal associated with a polydentate ligand. The polydentate ligand stabilizes the nanocrystal.

CLAIM OF PRIORITY

This application claims priority to provisional U.S. Patent ApplicationSer. No. 60/403,367, filed on Aug. 15, 2002, the entire contents ofwhich are hereby incorporated by reference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government may have certain rights in this invention pursuantto Contract No. N00014-01-1-0787 awarded by the Office of NavalResearch.

TECHNICAL FIELD

The invention relates to stabilized semiconductor nanocrystals.

BACKGROUND

Semiconductor nanocrystals have been a subject of great interest,promising extensive applications including display devices, informationstorage, biological tagging materials, photovoltaics, sensors andcatalysts. Nanocrystals having small diameters can have propertiesintermediate between molecular and bulk forms of matter. For example,nanocrystals based on semiconductor materials having small diameters canexhibit quantum confinement of both the electron and hole in all threedimensions, which leads to an increase in the effective band gap of thematerial with decreasing crystallite size. Consequently, both theoptical absorption and emission of nanocrystals shift to the blue (i.e.,to higher energies) as the size of the crystallites decreases.Semiconductor nanocrystals can have a narrow fluorescence band whoseemission wavelength is tunable with the size and material of thenanocrystals.

Nanocrystals consist of an inorganic nanoparticle that is surrounded bya layer of organic ligands. This organic ligand shell is critical to thenanocrystals for processing, binding to specific other moieties, andincorporation into various substrates. Fluorescent nanocrystals are moststable and robust when there is an excess amount of passivating ligandsin solution. Monodentate alkyl phosphines and alkyl phosphine oxidespassivate nanocrystals efficiently. Note that the term phosphine willrefer to both phosphines and phosphine oxides below. Nanocrystals can bestored in their growth solution, which contains a large excess ofligands such as alkyl phosphines and alkyl phosphine oxides, for longperiods without noticeable degradation. For most applications,nanocrystals must be processed outside of their growth solution andtransferred into various chemical environments. However, nanocrystalsoften lose their high fluorescence or become irreversibly aggregatedwhen removed from their growth solution.

SUMMARY

In general, a semiconductor nanocrystal having a polydentate ligand onthe surface of the nanocrystal can be stabilized in comparison to ananocrystal having a monodentate ligand on the surface of thenanocrystal. Monodentate ligands can readily exchange and diminish orquench emission from the nanocrystal as a result of the exchange. Whennanocrystals with conventional monodentate ligands are diluted orembedded in a non-passivating environment (i.e. one where no excessligands are present), the nanocrystals tend to lose their highluminescence and their initial chemical inertness, as manifested by, forexample, an abrupt decay of luminescence, aggregation, and/or phaseseparation. The polydentate ligand can be a polyphosphine, apolyphosphine oxide, a polyphosphinic acid, or a polyphosphonic acid, ora salt thereof.

Advantageously, polydentate ligands, particularly oligomerizedpolydentate ligands such as polydentate oligomerized phosphine ligands,bind more strongly to the surface of the nanocrystal than monodentateligands. Polydentate ligands thus stabilize the nanocrystal, which canpreserve the high luminescence of as-grown nanocrystals. Polydentatephosphines can be more securely anchored onto the nanocrystal surfacethan bidentate thiols. In a tagging application, for example, they canensure more secure chemical attachments of tags to their targets. Inaddition, because of the affinity of the polydentate ligands for thenanocrystal, minimal amounts of oligomeric phosphines can be used topassivate nanocrystals since the higher affinity and compatibilityensures a high local concentration of the ligand around the nanocrystalsurface. The polydentate ligand provides a local environment that isvery similar to its growth solution because the growth solution is themedium where the nanocrystal is most stable. The polydentate phosphineprovides a high density phosphine ligand layer on the nanocrystalsurface. Also advantageously, the outer portion of the polydentateligand, can be chosen to be compatible with the bulk environmentsurrounding the nanocrystal, such as an organic solvent, aqueous media,or polymer matrix. The polydentate ligands are chemically flexible sothat they can be easily functionalized to be compatible with a varietyof chemical environments. For example, the polydentate ligands can befunctionalized to be hydrophobic, hydrophilic, or polymerizable.

In one aspect, a semiconductor nanocrystal includes a semiconductornanocrystal and an outer layer comprising a polydentate ligand bonded tothe nanocrystal by three or more donor groups, each donor groupindependently selected from the group consisting of P, N, P═O, and N═O.The polydentate ligand can be a member of a distribution of oligomers.In another aspect, a semiconductor nanocrystal includes a semiconductornanocrystal, and an outer layer including a plurality of polydentateligands, each polydentate ligand bound to the nanocrystal by three ormore donor groups, each donor group independently selected from thegroup consisting of P, N, P═O, and N═O, the plurality of polydentateligands being a distribution of oligomers.

In another aspect, a semiconductor nanocrystal includes a semiconductornanocrystal and an outer layer including a polydentate ligand bound tothe nanocrystal by three or more donor groups, each donor groupindependently selected from the group consisting of P, N, P═O, and N═O,wherein the luminescence of the nanocrystal decreases by no more than50% after incubating for 24 hours in fetal bovine serum maintained at37° C.

In another aspect, a method of making a stabilized nanocrystal includescontacting a nanocrystal with a polydentate ligand having three or moredonor groups, each donor group independently selected from the groupconsisting of P, N, P═O, and N═O, to form the stabilized nanocrystal.Stabilizing the nanocrystals can include cross-linking the polydentateligand. The polydentate ligand can include a carboxylic acid, andcross-linking can include contacting the polydentate ligand with adiamine and a coupling agent. The polydentate ligand can include anacrylate group, and cross-linking can include contacting the polydentateligand with a radical initiator.

In another aspect, a method of making a polydentate ligand includescontacting a monomeric, polyfunctional phosphine with a polyfunctionaloligomerization reagent to form an oligomeric phosphine. The monomeric,polyfunctional phosphine can be trishydroxypropylphosphine. Thepolyfunctional oligomerization reagent can be a diisocyanate. Theoligomeric phosphine can be contacted with an isocyanate of formulaR′—L—NCO, wherein L is C₂–C₂₄ alkylene, and R′ has the formula

R′ has the formula

or R′ is hydrogen, wherein R^(a) is hydrogen or C₁–C₄ alkyl.

In yet another aspect, a method of making a nanocrystal-biomoleculeconjugate includes contacting a nanocrystal including a polydentateligand including a reactive group with a biomolecule. The biomoleculecan be a polypeptide. The nanocrystal and the biomolecule can becontacted with a cross-linking agent. The reactive group can be acarboxylic acid. The biomolecule can include an amino group and thecross-linking agent can be a carbodiimide.

The first semiconductor material can be a Group II–VI compound, a GroupII–V compound, a Group III–VI compound, a Group III–V compound, a GroupIV–VI compound, a Group I–III–VI compound, a Group II–IV–VI compound, ora Group II–IV–V compound, such as, for example, ZnS, ZnSe, ZnTe, CdS,CdSe, CdTe, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb,GaSe, InN, InP, InAs, InSb, TlN, TlP, TiAs, TlSb, PbS, PbSe, PbTe, ormixtures thereof. Each first semiconductor material can be overcoatedwith a second semiconductor material, such as ZnO, ZnS, ZnSe, ZnTe, CdO,CdS, CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, AlN, AlP,AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, TlN, TlP, TlAs,TlSb, TlSb, PbS, PbSe, PbTe, or mixtures thereof. The nanocrystal can bea member of a monodisperse distribution of sizes of nanocrystals. Thefirst semiconductor material can have a smaller band gap than the secondsemiconductor material.

Other features, objects, and advantages of the invention will beapparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram depicting representative chemical structures ofoligomeric phosphines.

FIG. 2 is a graph depicting the mass spectrum of oligomeric phosphine.

FIG. 3 is a set of graphs depicting quantum yield changes over time ofidentical CdSe/ZnS nanocrystals passivated by different ligands.

DETAILED DESCRIPTION

Nanocrystal cores can be prepared by the pyrolysis of organometallicprecursors in hot coordinating agents. See, for example, Murray, C. B.,et al., J. Am. Chem. Soc. 1993, 115, 8706, and Mikulec, F., Ph.D.Thesis, MIT, Cambridge, 1999, each of which is incorporated by referencein its entirety. Growth of shell layers on the bare nanocrystal corescan be carried out by simple modifications of conventional overcoatingprocedures. See, for example, Peng, X., et al., J. Am. Chem. Soc. 1997,119, 7019, Dabbousi, B. O., et al., J. Phys. Chem. B 1997, 101, 9463,and Cao, Y. W. and Banin, U. Angew. Chem. Int. Edit. 1999, 38, 3692,each of which is incorporated by reference in its entirety.

A coordinating agent can help control the growth of the nanocrystal. Thecoordinating agent is a compound having a donor lone pair that, forexample, has a lone electron pair available to coordinate to a surfaceof the growing nanocrystal. The coordinating agent can be a solvent. Acoordinating agent can stabilize the growing nanocrystal. Typicalcoordinating agents include alkyl phosphines, alkyl phosphine oxides,alkyl phosphonic acids, or alkyl phosphinic acids, however, othercoordinating agents, such as pyridines, furans, and amines may also besuitable for the nanocrystal production. Examples of suitablecoordinating agents include pyridine, tri-n-octyl phosphine (TOP) andtri-n-octyl phosphine oxide (TOPO). Technical grade TOPO can be used.

The outer surface of the nanocrystal can include a layer of compoundsderived from the coordinating agent used during the growth process. Thesurface can be modified by repeated exposure to an excess of a competingcoordinating group to form an overlayer. For example, a dispersion ofnanocrystals capped with the coordinating agent used during growth canbe treated with a coordinating organic compound, such as pyridine, toproduce crystallites which disperse readily in pyridine, methanol, andaromatics but no longer disperse in aliphatic solvents. Such a surfaceexchange process can be carried out with any compound capable ofcoordinating to or bonding with the outer surface of the nanocrystal,including, for example, phosphines, thiols, amines and phosphates. Thenanocrystal can be exposed to short chain polymers which exhibit anaffinity for the surface and which terminate in a moiety having anaffinity for a suspension or dispersion medium. Such affinity improvesthe stability of the suspension and discourages flocculation of thenanocrystal.

Monodentate alkyl phosphines and alkyl phosphine oxides passivatenanocrystals efficiently. Note that the term phosphine will refer toboth phosphines and phosphine oxides below. Other conventional ligandssuch as thiols or phosphonic acids can be less effective thanmonodentate phosphines for maintaining the initial high nanocrystalluminescence over long periods. For example, the photoluminescence ofnanocrystals consistently diminishes or quenches after ligand exchangeswith thiols or phosphonic acid.

An excess of free monodentate phosphine ligands can maintain highnanocrystal luminescence. An excess of free phosphine ligands can favora nanocrystal surface that is densely covered by the passivatingligands. When nanocrystals with conventional monodentate ligands arediluted or embedded in a non-passivating environment (i.e. anenvironment where excess ligands are not present), however, thenanocrystals can lose their high luminescence and chemical inertness. Insuch an environment, typical effects can include an abrupt loss ofluminescence, aggregation, and/or phase separation.

In order to overcome the limitations of monodentate ligands, polydentateligands, such as a distribution of oligomeric polydentate phosphineligands, can be used. Polydentate ligands show a high affinity for thenanocrystal surface. In other words, a polydentate ligand can have alarger equilibrium constant for binding to a nanocrystal than achemically similar monodentate ligand. Oligomeric phosphines have morethan one binding site to the nanocrystal surface, which contributes totheir high affinity for the nanocrystal surface. Oligomeric phosphinescan be preferred to bidentate thiols as nanocrystal ligands becauseoligomeric phosphines can preserve the high luminescence of as-grownnanocrystals. Moreover, polydentate phosphines can be more securelyanchored onto (i.e., have a higher affinity for) the nanocrystal surfacethan bidentate thiols. In a tagging application, for example, thepolydentate ligand can ensure a more secure chemical attachment of a tagto its target that a monodentate ligand. Minimal amounts of oligomericphosphines can be used to passivate nanocrystals. Unlike monodentateligands, an excess of oligomeric phosphines is not necessary to maintainthe high luminescence of nanocrystals. Oligomeric phosphines can providethe nanocrystal surface with a local environment that is very similar toits growth solution, where the nanocrystal is most stable. Polydentatephosphines can form a high-density phosphine ligand layer on thenanocrystal surface. To prevent aggregation or phase separation ofnanocrystals, the outermost surface of nanocrystal must be compatible tothe bulk environment. The ligands can be easily functionalized to becompatible with a variety of chemical environments. For instance, theycan be functionalized to be hydrophobic, hydrophilic, or polymerizable.

The polydentate ligand can be an oligomer, or a distribution ofoligomers. The polydentate ligand can have the formula:

where n is 1, 2, 3, 4 or 5, m is 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, eachk is 1, 2, 3, or 4, each X is N P, P═O or N═O, each Y is substituted orunsubstituted alkyl, substituted or unsubstituted alkoxy, substituted orunsubstituted aryl, or substituted or unsubstituted aryloxy, and L is alinking group optionally terminated by O and includes at least onecarbonate, carbamate, amide, ester or ether linkage.

The polydentate ligand can be of the formula:

where n is 1, 2 or 3, m is 1, 2, 3, 4, or 5, each k is 1 or 2, each X isN, P, P═O or N═O, each Y is substituted or unsubstituted alkyl,substituted or unsubstituted alkoxy, substituted or unsubstituted aryl,or substituted or unsubstituted aryloxy, and L is a linking groupoptionally terminated by O and includes at least one carbonate,carbamate, amide, ester or ether linkage.

The polydentate ligand can have the formula:

where p is 1 or 2, each m is 1, 2, 3, 4, or 5, each k is or 2, each j is0 or 1, each p is 0 or 1, q is 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, each Xis N, P, P═O or N═O, each Y is substituted or unsubstituted alkyl,substituted or unsubstituted alkoxy, substituted or unsubstituted aryl,or substituted or unsubstituted aryloxy, and L is a linking groupoptionally terminated by O and includes at least one carbonate,carbamate, amide, ester or ether linkage.

In certain circumstances, X is P or P═O, and L includes at least oncarbamate linkage. In certain circumstances, each Y can be unsubstitutedalkyl, each Y can include a carboxylic acid, or each Y can include anacrylate group.

The polydentate ligand can have the formula:

where n is 1, 2 or 3, m is 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, each k is 1or 2, each x independently is 0 or 1, each of Z¹ and Z², independently,is an ether, amide, ester, carbamate or carbonate linkage, each R¹ andR², independently, is an alkylene optionally interrupted by S, O, NH,N-lower alkyl, arylene, heteroarylene, or aralkylene and optionallyterminated by S, O, NH, N-lower alkyl, arylene, heteroarylene, oraralkylene, and each R is substituted or unsubstituted alkyl,substituted or unsubstituted alkoxy, or substituted or unsubstitutedaryl. In certain embodiments, Z¹ and Z² are each a carbamate linkage. Incertain circumstances, R¹ and R² are each an alkylene.

The polydentate ligand can have the formula:

where n is 1, 2 or 3, m is 1, 2, 3, 4, or 5, each k is 1 or 2, each xindependently is 0 or 1, Z is an ether, carbamate, amide, ester orcarbonate linkage, each R¹ and each R², independently, is an alkyleneoptionally interrupted by S, O, NH, N-lower alkyl, arylene,heteroarylene, or aralkylene, and optionally terminated by S, O, NH,N-lower alkyl, arylene, heteroarylene, or aralkylene, and each R issubstituted or unsubstituted alkyl, or substituted or unsubstitutedaryl, and each R is bonded to R¹ via an ether, ester, amide, carbamateor carbonate linkage.

The polydentate ligand can have the formula:

where n is 1, 2 or 3, m is 1, 2, 3, 4, or 5, each k is 1 or 2, each xindependently is 0 or 1, and each R is substituted or unsubstitutedalkyl, substituted or unsubstituted alkoxy, or substituted orunsubstituted aryl. R can have the formula:

The polydentate ligand can be cross-linked once bound to a nanocrystal.The cross-linked polydentate ligand can have the formula:

where each n independently is 1, 2 or 3, each m independently is 1, 2,3, 4, or 5, each k is 1 or 2, each X is N, P, P═O or N═O, each Y issubstituted or unsubstituted alkyl, substituted or unsubstituted alkoxy,substituted or unsubstituted aryl, or substituted or unsubstitutedaryloxy, L is a linking group optinally terminated by O and includes atleast one carbonate, carbamate, amide, ester or ether linkage, L′ is abond or a cross-linking group, and Y′—L′—Y′ is derived fromcross-linking of Y.

The cross-linked polydentate ligand can have the formula:

where n is 1, 2 or 3, m is 1, 2, 3, 4, or 5, each k is 1 or 2, each x is0 or 1, and each R is substituted or unsubstituted alkyl, or substitutedor unsubstituted aryl, L′ is a bond or a cross-linking group, andR′—L′—R′ is derived from cross-linking of R. When each R includes acarboxylic acid, the polydentate ligand can be cross-linked with, forexample, a diamine, and R′—L′—R′ can include the fragment:

where A is alkylene or arylene. When each R includes an acrylate group,the polydentate ligand can be cross-linked by radical polymerization ofthe acrylate groups, and R′—L′—R′ can include the fragment:

where A′ is H or C₁–C₄ alkyl.

FIG. 1 shows chemical structures of representative oligomeric phosphineswith functionalized branches. The exemplary functional groups shown arealkyl, methacrylate, and carboxylic acid. Many other functional groupscan be introduced with minor modifications to the synthesis. Thisflexibility can allow homogeneous incorporation of nanocrystals in anydesired medium.

The oligomeric ligands can create a trilayer around the nanocrystal: aphosphine layer, a hydrophobic linking layer, and a functionalizedlayer. The phosphine layer can passivate the nanocrystal surface, thehydrophobic layer can protect it, while the functionalized layer candeliver desirable chemical properties including solubility, miscibility,the ability to copolymerize with other matrices, further cross-linkingon the surface of the nanocrystals, and other derivatizations such asconjugation to biomolecules.

The synthesis of oligomeric phosphines (such as those shown in FIG. 1)and methods for ligand exchange on nanocrystal surfaces are describedbelow. The synthesis is flexible and can be easily modified. In general,a monomeric phosphine is oligomerized, and the resulting oligomericphosphine is functionalized. A specific example is shown in Scheme 1,which can be easily generalized and modified to synthesize thepolydentate ligands described here. As shown in Scheme 1, a monomericphosphine such as trishydroxypropylphosphine (THPP) can be oligomerizedby reaction with a multifunctional linker such as diisocyanatohexane(DIH). Though Scheme 1 shows a linear oligomer, branched oligomers arepossible. The linker can be a bifunctional, trifunctional or higherfunctional linker. The distribution of oligomers can be controlled byadjusting the stoichiometry of the monomeric unit and linker. In certaincircumstances, the distribution of oligomers includes primarilyoligomers with n=1, 2, 3, or 4 according to Scheme 1. Many other linkerscan also be used. Various alkyldiisocyanates with different length alkylchains and aryldiisocyanates are commercially available (for example,from Sigma-Aldrich) and can act as varying length spacers betweenphosphine groups within the oligomers.

The oligomeric phosphine can be functionalized, for example by reactionwith a second isocyanate including a group that bestows a desiredproperty on the functionalized oligomeric phosphine. The secondisocyanate is represented in Scheme 1 as R—NCO. For example, if thedesired property is hydrophobicity, the second isocyanate can include ahydrophobic group such as an alkyl chain, as in octyl isocyanate orhexadecyl isocyanate. Other examples of properties that can beintroduced include hydrophilicity (e.g. from a hydrophilic group such asa carboxylic acid) and ability to polymerize (e.g. from a polymerizablegroup such as an acrylate or methacrylate). See FIG. 1. In somecircumstances, the ligand can be exposed to oxygen (for example, air) tooxidize the donor atoms (i.e. P or N).

Chemical functionality can be introduced to the small oligomericphosphine by further reactions with any molecule or a combination ofmolecules. The functionality can be introduced, for example, by reactionof an oligomeric phosphine having unreacted hydroxyl groups with amolecule having a desired functional group and an isocyanate group. SeeScheme 1. For example, octylisocyanate or hexadecylisocyanate can beused to introduce a hydrophobic alkyl chain, and a polymerizablemethacrylate group can be introduced by reaction with2-isocyanatoethylmethacrylate. In some cases, conventional protectionand deprotection procedures on the desired functional group may benecessary to facilitate synthesis. An oligomeric phosphine bearingcarboxylic acid groups (FIG. 1) can be prepared by hydrolysis of anester derivatized oligomeric phosphine. The ester derivatized oligomericphosphine can prepared from the reaction between the oligomericphosphine and methyl-5-isocyanatopentanoate. Advantageously, the estercan be selectively hydrolyzed under basic hydrolysis conditions whileretaining the carbamate linkages.

Carbamate bond formation between a monomeric phosphine, such as THPP,and a diisocyanate such as DIH can be advantageous as an oligomerizationreaction. Advantages of this oligomerization reaction include a reactionto completeness under mild conditions at room temperature. The monomericphosphine, in addition to serving as a reactant, can catalyze thecarbamate bond formation reaction. Tin compounds such as dibutyltindilaurate can be added to further catalyze the reaction. See, forexample, Ulrich, H., Chemistry and technology of isocyanates 1996,Chichester, N.Y. , J. Wiley & Sons, which is incorporated by referencein its entirety. Another advantage is the small extent of sidereactions, such that purification can be unnecessary. An additionaladvantage is that the carbamate bond can be stable enough for mostpurposes such as fluorescence in situ hybridization procedures. See, forexample, Pathak, S., et al., 2001 J. Am. Chem. Soc. 123, 4103, and Palm,V. A., Tables of rate and equilibrium constants of heterolytic organicreactions V.1 1975 Laboratory of chemical kinetics and catalysis atTartu State University, Moscow, each of which is incorporated byreference in its entirety.

In one example of a polydentate ligand, FIG. 2 shows a mass spectrum ofan unfunctionalized oligomeric phosphine, and reveals a narrowdistribution of oligomers. Labels a), b), c) and d) indicate peaks thatcorrespond to the oligomeric phosphine depicted in Scheme 1, with n=1,n=2, n=3, and n=4, respectively. The mass spectrum was recorded with aBruker Daltonics APEX3 with an electrospray ionization source. Peaksfrom multiple charges were deconvoluted to singly charged mass numbersto demonstrate the distribution of oligomers.

Ligand exchanges (e.g. substitution of an oligomeric phosphine for amonodentate phosphine) can be carried out by one-phase or two-phasemethods. Prior to ligand exchange, nanocrystals can be precipitated fromtheir growth solutions by addition of methanol. The supernatantsolution, which includes excess coordinating agent (e.g.,trioctylphosphine), can be discarded. The precipitated nanocrystals canbe redispersed in hexanes. Precipitation and redispersion can berepeated until essentially all the excess coordinating agent has beenseparated from the nanocrystals. A one-phase process can be used whenboth the nanocrystals and the ligands to be introduced are soluble inthe same solvent. A solution with an excess of new ligands can be mixedwith the nanocrystals. The mixture can be stirred at an elevatedtemperature until ligand exchange is complete. The one-phase method canbe used, for example, to exchange octyl-modified oligomeric phosphinesor methacrylate-modified oligomeric phosphines, which are both solublein solvents that are compatible with the nanocrystals, such as hexanes.A two-phase ligand exchange process can be preferable when thenanocrystals and the new ligands do not have a common solvent.Nanocrystals can dissolved in an organic solvent such asdichloromethane, and the new ligand can be dissolved in an aqueoussolution. The nanocrystals can be transferred from the organic phase tothe aqueous phase by, for example, sonication. The transfer can bemonitored through absorption and emission spectroscopy. A carboxylicacid-modified oligomeric phosphine can be introduced to nanocrystals viathis method. A similar two-phase ligand exchange process has beenreported earlier. See, for example, Wang, Y. A., et al., 2002 J. Am.Chem. Soc 124, 2293, incorporated by reference in its entirety.

FIG. 3 shows a comparison of nanocrystal stability in the presence ofoligomeric phosphine ligands or monomeric ligands. The comparison wasmade in organic solvent and in aqueous solution. Equimolar binding sites(i.e. phosphine or thiol moieties) were used, with only a slight excessof ligand present relative to the concentration of nanocrystal. Thisensures that there are very small amounts of extra free ligands in thesolution. Therefore, the stabilities of photoluminescence can bevalidated as a method to measure the different binding affinities andpassivating powers of the ligands on nanocrystal surface. The top panelshows that nanocrystals dispersed in THF, passivated by oligomericphosphine with hexadecyl alkyl chain (solid line) are more stable thanthose passivated by trioctylphosphine (dotted line). The bottom panelshows that, in aqueous 0.1 M potassium hydroxide, nanocrystalspassivated by oligomeric phosphine with carboxylic acid (solid line) aregreatly stabilized compared to nanocrystals passivated bymercaptoundecanoic acid (dotted line).

In certain circumstances, a functionalized oligomeric phosphine can becross-linked once bound to the nanocrystal. Such cross-linking canfurther increase the stability of the nanocrystals. Cross-linking can beaccomplished by, for example, addition of a diamine such as2,6-diaminopimelic acid a carbodiimide dehydrating agent to carboxylicacid-functionalized oligomeric phosphine. Cross-linking can be carriedout while the ligand is bound to a nanocrystal. Another example ofcross-linking is the radical polymerization of the methacrylate groupsof a methacrylate-modified oligomeric phosphine.

Nanocrystals with oligomeric phosphine ligands can be conjugated tobiomolecules. For example, nanocrystals having carboxylic acid-modifiedoligomeric phosphine ligands can be coupled to biomolecules containingamino groups. The coupling can be facilitated by a carbodiimidedehydrating agent, such as EDC(1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride). Thegeneral coupling reaction is described, for example, in Hermanson, G. T.Bioconjugate Techniques 1996 Academic Press, which is incorporated byreference in its entirety. Electrostatic interactions can be also usedas thiol-based ligands with carboxylic acid. See, for example,Mattoussi, H., et al., J. Am. Chem. Soc. 2000, 122, 12142, and Goldman,E. R., et al., 2002 J. Am. Chem. Soc. 124, 6378, each of which isincorporated by reference in its entirety. Additional cross-linkingagents that can couple nanocrystals with oligomeric phosphine ligands tobiomolecules include carbonyldiimidazole and epichlorohydrin. See, forexample, Pathak S., et al., 2001 J. Am. Chem. Soc 123, 4103, andHermanson, G. T. Bioconjugate Techniques 1996 Academic Press, each ofwhich is incorporated by reference in its entirety.

The nanocrystal can be a member of a population of nanocrystals having anarrow size distribution. The nanocrystal can be a sphere, rod, disk, orother shape. The nanocrystal can include a core of a semiconductormaterial. The nanocrystal can include a core having the formula MX,where M is cadmium, zinc, magnesium, mercury, aluminum, gallium, indium,thallium, or mixtures thereof, and X is oxygen, sulfur, selenium,tellurium, nitrogen, phosphorus, arsenic, antimony, or mixtures thereof.

The semiconductor forming the core of the nanocrystal can include GroupII–VI compounds, Group II–V compounds, Group III–VI compounds, GroupIII–V compounds, Group IV–VI compounds, Group I–III–VI compounds, GroupII–IV–VI compounds, and Group II–IV–V compounds, for example, ZnS, ZnSe,ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP,GaAs, GaSb, GaSe, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, PbS, PbSe,PbTe, or mixtures thereof.

The core can have an overcoating on a surface of the core. Theovercoating can be a semiconductor material having a compositiondifferent from the composition of the core. The overcoat of asemiconductor material on a surface of the nanocrystal can include aGroup II–VI compounds, Group II–V compounds, Group III–VI compounds,Group III–V compounds, Group IV–VI compounds, Group I–III–VI compounds,Group II–IV–VI compounds, and Group II–IV–V compounds, for example, ZnS,ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, AiN, AlP, AlAs, AlSb, GaN,GaP, GaAs, GaSb, GaSe, InN, InP, InAs, InSb, TiN, TiP, TiAs, TlSb, PbS,PbSe, PbTe, or mixtures thereof. The overcoating material can have aband gap greater than the band gap of the core material. Alternatively,the overcoating material can have a band (i.e. the valence band or theconduction band) intermediate in energy to the valence and conductionbands of the core material. See for example, U.S. patent application No.10/638,546, titled, “Semiconductor Nanocrystal Heterostructures”, filedAug. 12, 2003, which is incorporated by reference in its entirety.

The emission from the nanocrystal can be a narrow Gaussian emission bandthat can be tuned through the complete wavelength range of theultraviolet, visible, or infrared regions of the spectrum by varying thesize of the nanocrystal, the composition of the nanocrystal, or both.For example, CdSe can be tuned in the visible region and InAs can betuned in the infrared region.

The population of nanocrystals can have a narrow size distribution. Thepopulation can be monodisperse and can exhibit less than a 15% rmsdeviation in diameter of the nanocrystals, preferably less than 10%,more preferably less than 5%. Spectral emissions in a narrow range ofbetween 10 and 100 nm full width at half max (FWHM) can be observed.Semiconductor nanocrystals can have emission quantum efficiencies ofgreater than 2%, 5%, 10%, 20%, 40%, 60%, 70%, or 80%.

Methods of preparing semiconductor nanocrystals include pyrolysis oforganometallic reagents, such as dimethyl cadmium, injected into a hot,coordinating agent. This permits discrete nucleation and results in thecontrolled growth of macroscopic quantities of nanocrystals. Preparationand manipulation of nanocrystals are described, for example, in U.S.Pat. No. 6,322,901, incorporated herein by reference in its entirety.The method of manufacturing a nanocrystal is a colloidal growth processand can produce a monodisperse particle population. Colloidal growthoccurs by rapidly injecting an M donor and an X donor into a hotcoordinating agent. The injection produces a nucleus that can be grownin a controlled manner to form a nanocrystal. The reaction mixture canbe gently heated to grow and anneal the nanocrystal. Both the averagesize and the size distribution of the nanocrystals in a sample aredependent on the growth temperature. The growth temperature necessary tomaintain steady growth increases with increasing average crystal size.The nanocrystal is a member of a population of nanocrystals. As a resultof the discrete nucleation and controlled growth, the population ofnanocrystals obtained has a narrow, monodisperse distribution ofdiameters. The monodisperse distribution of diameters can also bereferred to as a size. The process of controlled growth and annealing ofthe nanocrystals in the coordinating agent that follows nucleation canalso result in uniform surface derivatization and regular corestructures. As the size distribution sharpens, the temperature can beraised to maintain steady growth. By adding more M donor or X donor, thegrowth period can be shortened.

An overcoating process is described, for example, in U.S. Pat. No.6,322,901, incorporated herein by reference in its entirety. Byadjusting the temperature of the reaction mixture during overcoating andmonitoring the absorption spectrum of the core, over coated materialshaving high emission quantum efficiencies and narrow size distributionscan be obtained.

The M donor can be an inorganic compound, an organometallic compound, orelemental metal. The inorganic compound M-containing salt can be a metalhalide, metal carboxylate, metal carbonate, metal hydroxide, or metaldiketonate, such as a metal acetylacetonate. See, for example, U.S. Pat.No. 6,576,291, which is incorporated by reference in its entirety. M iscadmium, zinc, magnesium, mercury, aluminum, gallium, indium orthallium. The X donor is a compound capable of reacting with the M donorto form a material with the general formula MX. Typically, the X donoris a chalcogenide donor or a pnictide donor, such as a phosphinechalcogenide, a bis(silyl) chalcogenide, dioxygen, an ammonium salt, ora tris(silyl) pnictide. Suitable X donors include dioxygen,bis(trimethylsilyl) selenide ((TMS)₂Se), trialkyl phosphine selenidessuch as (tri-n-octylphosphine) selenide (TOPSe) or(tri-n-butylphosphine) selenide (TBPSe), trialkyl phosphine telluridessuch as (tri-n-octylphosphine) telluride (TOPTe) orhexapropylphosphorustriamide telluride (HPPTTe),bis(trimethylsilyl)telluride ((TMS)₂Te), bis(trimethylsilyl)sulfide((TMS)₂S), a trialkyl phosphine sulfide such as (tri-n-octylphosphine)sulfide (TOPS), an ammonium salt such as an ammonium halide (e.g.,NH₄Cl), tris(trimethylsilyl) phosphide ((TMS)₃P), tris(trimethylsilyl)arsenide ((TMS)₃As), or tris(trimethylsilyl) antimonide ((TMS)₃Sb). Incertain embodiments, the M donor and the X donor can be moieties withinthe same molecule.

Size distribution during the growth stage of the reaction can beestimated by monitoring the absorption line widths of the particles.Modification of the reaction temperature in response to changes in theabsorption spectrum of the particles allows the maintenance of a sharpparticle size distribution during growth. Reactants can be added to thenucleation solution during crystal growth to grow larger crystals. Bystopping growth at a particular nanocrystal average diameter, apopulation having an average nanocrystal diameter of less than 150 Å canbe obtained. A population of nanocrystals can have an average diameterof 15 Å to 125 Å.

The particle size distribution can be further refined by size selectiveprecipitation with a poor solvent for the nanocrystals, such asmethanol/butanol as described in U.S. Pat. No. 6,322,901, incorporatedherein by reference in its entirety. For example, nanocrystals can bedispersed in a solution of 10% butanol in hexane. Methanol can be addeddropwise to this stirring solution until opalescence persists.Separation of supernatant and flocculate by centrifugation produces aprecipitate enriched with the largest crystallites in the sample. Thisprocedure can be repeated until no further sharpening of the opticalabsorption spectrum is noted. Size-selective precipitation can becarried out in a variety of solvent/nonsolvent pairs, includingpyridine/hexane and chloroform/methanol. The size-selected nanocrystalpopulation can have no more than a 15% rms deviation from mean diameter,preferably 10% rms deviation or less, and more preferably 5% rmsdeviation or less.

Transmission electron microscopy (TEM) can provide information about thesize, shape, and distribution of the nanocrystal population. PowderX-ray diffraction (XRD) patterns can provided the most completeinformation regarding the type and quality of the crystal structure ofthe nanocrystals. Estimates of size are also possible since particlediameter is inversely related, via the X-ray coherence length, to thepeak width. For example, the diameter of the nanocrystal can be measureddirectly by transmission electron microscopy or estimated from X-raydiffraction data using, for example, the Scherrer equation. It also canbe estimated from the UV/Vis absorption spectrum.

EXAMPLES

All the procedures described here are carried out under an inertatmosphere unless specified otherwise. All commercial chemicals are useddirectly without any purification.

Oligomeric phosphines were synthesized by polymerizing an alkylphosphine, which was further functionalized in a subsequent reaction.Oligomeric phosphines refer to a distribution of oligomerizedphosphines. The distribution of oligomerized phosphines inlcudesprimarily of oligomers with n=1, 2, 3, and 4 (see Scheme 1).

Oligomeric phosphines were synthesized as follows.Trishydroxypropylphosphine (8.00 g) (THPP, Strem, 90%) of was dissolvedin 20.0 g of dimethylformamide (DMF, Aldrich, 99.8%). Diisocyanatohexane(4.54 g) (DIH, Aldrich, 98%) was added dropwise while the solution wasvigorously stirred. After the addition was complete, the solution wasstirred overnight. The solvent was removed at a reduced pressure and themixture was characterized by mass spectroscopy. ESI-MS(m/z): exp.961.6(M+H⁺), calc. 961.6 for n=1 in Scheme 1, exp. 1337.9(M+H⁺), calc.1337.8 for n=2 in Scheme 1, exp. 1713.9(M+H⁺), calc. 1714.0 for n=3 inScheme 1, exp. 2090.3(M+H⁺), calc. 2090.2 for n=4 in Scheme 1 See FIG.2.

Oligomeric phosphines were functionalized with octyl alkyl chains toform octyl-modified oligomeric phosphines. The octyl-modified oligomericphosphines are compatible with hydrophobic environments, and afterexchange with the existing surface capping groups, can render thenanocrystals compatible also with many hydrophobic environments.

The octyl-modified oligomeric phosphines were synthesized as follows.Oligomeric phosphines (2.86 g, prepared as above) were dissolved in 3.0mL of DMF. Octylisocyanate (2.31 g) (Aldrich, 97%) was added dropwise.After the addition was complete, the solution was stirred overnight. Thesolvent was removed at a reduced pressure. The mixture was characterizedby mass spectroscopy. ESI-MS(m/z): exp. 1737.2(M+H⁺), calc. 1737.2 forn=1 in Scheme 1, exp. 2268.6(M+H⁺), calc. 2268.6 for n=2 in Scheme 1.

The oligomeric phosphines were exchanged with the nanocrystal surfacecapping groups as follows. CdSe/ZnS nanocrystal powder free of excesstrioctylphosphine oxide was obtained by nonsolvent-precipitation methodsfrom 0.1 mL growth solution. Octyl-modified oligomeric phosphines (0.2mL) in DMF solution (64% wt/wt) and 3.0 mL of THF were added to thenanocrystal powder and stirred vigorously at 60° C. for overnight. Theresultant nanocrystals were now capped with the octyl-modifiedoligomeric phosphine ligands. During the steps described above, anexcess amount of new ligands were used to complete the ligand-exchange.The excess ligands were removed by precipitation followed byultra-centrifugation. The precipitation can be induced by the additionof methanol to the solution.

Hexadecyl-modified oligomeric phosphines were also prepared that werecompatible with many hydrophobic environments, and after exchange withsurface capping groups, rendered the nanocrystals compatible with manyhydrophobic environments. These ligands were prepared in the same manneras the small oligomeric phosphines with octyl alkyl chains except 3.98 gof hexadecylisocyanate (Aldrich, 97%) were used in place of 2.31 goctylisocyanate.

Methacrylate-modified oligomeric phosphines can allow nanocrystals to beincorporated into polymer media by co-polymerization, which can reduceor prevent the occurrence of phase separation of nanocrystals. Themethacrylate-modified oligomeric phosphines were prepared as follows.Oligomeric phosphines (3.0 g) of in DMF solution (40% wt/wt) werediluted by 6.0 mL of DMF. The solution was stirred vigorously in an icebath while 0.97 g of 2-isocyanatoethylmethacrylate (Aldrich, 98%) wasslowly added for 4 hours. After the addition, the solution was stirredin the ice bath overnight. The solvent was removed at a reducedpressure.

The exchange of capping groups was carried out as follows. CdSe/ZnSnanocrystal powder free of excess trioctylphosphine oxide was obtainedby standard nonsolvent-precipitation methods from 0.1 mL growthsolution. Oligomeric phosphines with methacrylate in DMF solution (40%wt/wt, 0.3 mL) was added to the nanocrystals and stirred vigorouslyovernight. The nanocrystals were now capped with the new ligand andpossess the methacrylate functionality for further chemistry. During thesteps above, an excess amount of new ligands were used to complete theligand-exchange. The excess ligands were removed by precipitationfollowed by ultra-centrifugation. The precipitation can be induced by anaddition of acetonitrile.

Oligomeric phosphines with carboxylic acid are compatible with aqueousenvironments, including biological environments. The carboxylic acid isavailable for further standard coupling chemistries. The smalloligomeric phosphines with carboxylic acid was prepared as follows.Oligomeric phosphines (0.16 g) were dissolved in 2.0 mL of DMF.Methyl-5-isocyanatopentanoate (0.26 g) (synthesis below) was addeddropwise. After the addition was complete, the solution was stirredovernight. The solvent was removed at a reduced pressure. Potassiumhydroxide (Mallinckrodt, 88%, 0.5 g), 2.0 mL of tetrahydrofuran(Aldrich, 99.9%) and 2.0 mL of distilled water were added and stirredvigorously at 60° C. for 1 day. The solvent was removed at a reducedpressure.

Methyl-5-isocyanatopentanoate was synthesized by combining 1.0 g ofmethyladipoylchloride (Lancaster, 96%), 0.4 g sodium azide (Aldrich,99%) and 4.0 mL of benzene (Aldrich, 99.8%) were mixed and stirred for 1day. The mixture was passed through a filter paper, and vacuumdistilled.

The oligomeric phosphines with carboxylic acid were exchanged withsurface capping groups as follows. Out of 1.0 mL growth solution, aCdSe/ZnS nanocrystal powder, free of excess trioctylphosphine oxide, wasobtained by nonsolvent-precipitation methods and dissolved in 3.0 mL ofdichloromethane (Aldrich, 99.6%). 10 mL of 0.2 M oligomeric phosphineswith carboxylic acid/KOH aqueous solution (described above) was added tothe powder. The mixture was sonicated overnight. The emulsified solutionwas separated into two different layers by centrifugation. The aqueouslayer was obtained by decanting after verifying that the nanocrystalswere completely transferred in. These nanocrystals were derivatized witha polyphosphine carboxylic acid. During the steps above, an excessamount of new ligands was used to complete the ligand-exchange. Theexcess ligands can be removed by dialysis, for example by repeateddilution and filtration using a membrane centrifugal dialysis kit ofnominal molecular cut-off of 50,000 daltons.

CdSe/ZnS(core/shell) nanocrystals were ligand-exchanged with oligomericphosphine with carboxylic acid as follows. Excess ligands wererigorously removed by repeated dialysis. A 0.1 M MES was introduced, andthe number of semiconductor nanocrystal particles in the solution wasdetermined by measuring the optical absorption. See Leatherdale, C. A.;Woo, W. K.; Mikulec, F. V.; Bawendi, M. G. Journal of Physical ChemistryB 2002, 106, 7619, which is incorporated by reference in its entirety.The carbodiimide cross-linking agent EDC(1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride, Pierce,25,000 equivalents) and 125,000 equivalents of N-hydroxysulfosuccinimide(Pierce) per nanocrystal were added to the nanocrystal solution. Thesolution was incubated for 15 minutes, and excess reagents were removedby dialysis in 0.1 M MES buffer. A PBS solution containing 5,000equivalents of 2,6-diaminopimelic acid (Aldrich, 98%) was mixed with theMES solution. The final pH was around 7.0 after mixing. The reactionsolution was incubated for 4 hours and the ligand-exchange andcross-linked nanocrystals were purified by repeated dialysis.

The stability of cross-linked nanocrystals bound by oligomeric phosphinewith carboxylic acid was compared to noncross-linked nanocrystals bymonitoring luminescence change over 100% fetal bovine serum at 37° C.over 24 hours. The cross-linked nanocrystals experienced less than 20%loss of luminescence, whereas the luminescence of noncross-linkednanocrystals decreased by more than 50%.

Streptavidin conjugation to CdSe/ZnS(core/shell) nanocrystals bound tooligomeric phosphine with carboxylic acid was carried out by a proceduresimilar to that described above for cross-linking. 100 equivalents ofstreptavidin (Pierce) were used instead of 2,6-diaminopimelic acid.Streptavidin conjugation can also be achieved with nanocrystalspreviously cross-linked by 2,6-diaminopimelic acid. Fluorescencemicrographs revealed that streptavidin conjugated nanocrystals boundspecifically to biotin-agarose beads, whereas nanocrystals notconjugated to streptavidin did not.

Oligomeric phosphines with methacrylate can enable homogeneousincorporation (i.e., co-polymerization) of nanocrystals into manypolymer matrices without the need for additional free ligands such asTOP in the matrix. The polymerizable ligands can become incorporatedinto host polymers and offer synthetic routes to micron and sub-micronsized polymer-nanocrystal composites. For example, fluorescent polymersticks incorporating semiconductor nanocrystals were prepared asfollows: CdSe/ZnS(core/shell) nanocrystals were ligand-exchanged witholigomeric phosphine with methacrylate and mixed with hydroxypropylmethacrylate (Aldrich, 97%), ethyleneglycol dimethacrylate (Aldrich,98%), and a small amount (<1% wt/wt) of 2,2′-azobisisobutyronirrile(Aldrich, 98%). The solution was transferred to a glass tube andpartially immersed in an oil bath at 70° C. until the polymerization wascomplete, ˜3 hours.

Other embodiments are within the scope of the following claims.

1. A polydentate ligand of formula:

wherein n is 1, 2, 3, 4 or 5, m is 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10,each k is 1, 2, 3, or 4, each X independently is a donor group selectedfrom the group consisting of N, P, P═O, and N═O, each Y is substitutedor unsubstituted alkyl, substituted or unsubstituted alkoxy, substitutedor unsubstituted aryl, or substituted or unsubstituted aryloxy, and L isa linking group optionally terminated by O and includes at least onecarbonate, carbamate, amide, ester or ether linkage.
 2. A polydentateligand of formula:

wherein n is 1, 2 or 3, m is 1, 2, 3,4, or 5, each k is 1 or 2, each Xindependently is a donor group selected from the group consisting of N,P, P═O, and N═O, each Y is substituted or unsubstituted alkyl,substituted or unsubstituted alkoxy, substituted or unsubstituted aryl,or substituted or unsubstituted aryloxy, and L is a linking groupoptionally terminated by O and includes at least one carbonate,carbamate, amide, ester or ether linkage.
 3. A distribution ofpolydentate ligands of formula:

wherein n is 1, 2 or 3, m is 1, 2, 3, 4, or 5, each k is 1 or 2, each Xindependently is a donor group selected from the group consisting of N,P, P═O, and N═O, each Y is substituted or unsubstituted alkyl,substituted or unsubstituted alkoxy, substituted or unsubstituted aryl,or substituted or unsubstituted aryloxy, L is a linking group optionallyterminated by O and includes at least one carbonate, carbamate, amide,ester or ether linkage, and the distribution includes at least twomembers with different values of m.
 4. A polydentate ligand of formula:

wherein p is 1 or 2, each m is 1, 2, 3, 4, or 5, each k is 1 or 2, eachj is 0 or 1, each p is 0 or 1, q is 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10,each X independently is a donor group selected from the group consistingof N, P, P═O, and N═O, each Y is substituted or unsubstituted alkyl,substituted or unsubstituted alkoxy, substituted or unsubstituted aryl,or substituted or unsubstituted aryloxy, and L is a linking groupoptionally terminated by O and includes at least one carbonate,carbamate, amide, ester or ether linkage.
 5. A polydentate ligand offormula:

wherein n is 1, 2 or 3, m is 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, each k is1 or 2, each x independently is 0 or 1, each of Z¹ and Z²,independently, is an ether, amide, ester, carbamate or carbonatelinkage, each R¹ and R², independently, is an alkylene optionallyinterrupted by S, O, NH, N-lower alkyl, arylene, heteroarylene, oraralkylene and optionally terminated by S, O, NH, N-lower alkyl,arylene, heteroarylene, or aralkylene, and each R is substituted orunsubstituted alkyl, substituted or unsubstituted alkoxy, or substitutedor unsubstituted aryl.
 6. A polydentate ligand of formula:

wherein n is 1, 2 or 3, m is 1, 2, 3, 4, or 5, each k is 1 or 2, each xindependently is 0 or 1, Z is an ether, carbamate, amide, ester orcarbonate linkage, each R¹ and each R², independently, is an alkyleneoptionally interrupted by S, O, NH, N-lower alkyl, arylene,heteroarylene, or aralkylene, and optionally terminated by S, O, NH,N-lower alkyl, arylene, heteroarylene, or aralkylene, and each R issubstituted or unsubstituted alkyl, or substituted or unsubstitutedaryl, and each R is bonded to R¹ via an ether, ester, amide, carbamateor carbonate linkage.
 7. A polydentate ligand of formula:

wherein n is 1, 2 or 3, m is 1, 2, 3, 4, or 5, each k is 1 or 2, each xindependently is 0 or 1, and each R is substituted or unsubstitutedalkyl, substituted or unsubstituted alkoxy, or substituted orunsubstituted aryl.
 8. The ligand of claim 7, wherein at least one x is0.
 9. The ligand of claim 7, wherein each x is
 0. 10. The ligand ofclaim 7, wherein each R is unsubstituted alkyl.
 11. The ligand of claim7, wherein each R includes a carboxylic acid group.
 12. The ligand ofclaim 7, wherein each R includes an acrylate group.
 13. A method ofmaking a polydentate ligand comprising contacting a monomeric,polyfunctional phosphine with a polyfunctional oligomerization reagentto form an oligomeric phosphine.
 14. The method of claim 13, wherein themonomeric, polyfunctional phosphine is trishydroxypropylphosphine. 15.The method of claim 14, wherein the polyfunctional oligomerizationreagent is a diisocyanate.
 16. The method of claim 13, furthercomprising contacting the oligomeric phosphine with an isocyanate offormula: R′—L—NCO, wherein L is C₂–C₂₄ alkylene, and R′ has the formula:

R′ has the formula:

or R′ is hydrogen, wherein R^(a) is hydrogen or C₁–C₄ alkyl.
 17. Amethod of making a nanocrystal-biomolecule conjugate comprisingcontacting a nanocrystal including a polydentate ligand including areactive group with a biomolecule.
 18. The method of claim 17, whereinthe biomolecule is a polypeptide.
 19. The method of claim 17, furthercomprising contacting the nanocrystal and the biomolecule with across-linking agent.
 20. The method of claim 19, wherein the reactivegroup is a carboxylic acid.
 21. The method of claim 20, wherein thebiomolecule includes an amino group and the cross-linking agent is acarbodiimide.